Generally, a polymer electrolyte fuel cell (PEFC) employs an electrolyte membrane. The electrolyte membrane is a polymer ion exchange membrane (proton ion exchange membrane). The electrolyte membrane is interposed between an anode and a cathode to form an assembly (electrolyte electrode assembly). Each of the anode and the cathode includes base material chiefly containing carbon, and an electrode catalyst layer of noble metal deposited on the base material. The electrolyte electrode assembly is sandwiched between separators (bipolar plates) to form a unit cell (unit power generation cell). In use, typically, a plurality of unit cells are stacked together to form a fuel cell stack.
In the fuel cell, a fuel gas such as a gas chiefly containing hydrogen (hereinafter also referred to as the hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (hereinafter also referred to as the oxygen-containing gas) is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
When the electrolyte membrane of the fuel cell is dried, it is not possible to maintain the operation at a high output density. Therefore, it is necessary to suitably humidify the electrolyte membrane. For this purpose, various humidification methods have been adopted conventionally. For example, in an external humidification method, the electrolyte membrane of the assembly is humidified by supplying water to the assembly using a humidifier such as a bubbler provided externally to the fuel cell. The humidifier humidifies reactant gases (fuel gas/oxygen-containing gas) supplied to the assembly. In an internal humidification method, a humidifier (humidification structure) for humidifying the electrolyte membrane is provided in the unit cell.
However, in the external humidification method, since the humidifier is provided externally to the fuel cell as an additional component, the fuel cell system is large as a whole. Thus, a large space is needed for the system. In particular, when the load of the fuel cell is increased rapidly, the humidifier may not have the capability for tracking the rapid increase of the load.
In one internal humidification method, strings for absorbing water are embedded in the electrolyte membrane. In another internal humidification method, water from the anode passes through a water permeable plate. In still another internal humidification method, water absorption strings are in contact with the electrolyte membrane on the anode side. However, in these methods, when the sufficient level of humidify is not achieved for some reasons, it is difficult to suitably recover the humidity in the fuel cell.